Deformation and Metamorphism in the Precambrians of Rajasthan

The varied lithological units of the Precambrian complex, represented by the Pre-Aravallis, Aravallis and Delhis, exhibit a set of complicated structural and metamorphic features that were the result of overprinting of at least four successive cycles of non-affine deformative movements, followed by later minor orogenic impulses spread over a long span of time. The Delhis are characterised by two sets of major folding movements giving rise to NNE-SSW (F₃) and WNW-ESE (F₄) structural trends, while the Aravallis are associated with two completely separate structural trends i.e. E-W (F₁) and NW-SE (F₂), which indicates that the tectonicity varied in space and also in time during the Delhi and Aravalli times. The fold-pattern in the Pre-Aravallis is the same as is noticed in the Aravallis.

The main metamorphic episodes in the Pre-Aravallis, Aravallis and Delhis, which were, sensu stricto, regional in nature, were coeval with the earlier phases of the major folding movements and were followed by dislocation and diaphthoresis in zones of shearing, thereby causing retrograde metamorphism in limited locales. The grade of regional metamorphism, in general, varies between lower greenschist facies to Upper amphibolite facies. The presence of granulites, at places, however, suggests higher temperature and pressure conditions prevailing at least locally. The thermal metamorphic effects in contact with basic and ultrabasic intrusives and with effusive acidic phases are very local.